34 A BICYCLE ERGOMETER WITH AN ELECTRIC BRAKE 



disk. Hence but few observations were made in the center of the field, 

 and no reliable ones were obtained there when the disk was rotating. 

 During the magnetic observations the ergometer was mounted inside the 

 calorimeter, but the front of the calorimeter was open and no attempt was 

 made to allow the thermal conditions to reach a steady state; each speed 

 was generally maintained for only a minute or less. Hence in general the 

 temperature of the disk was somewhat lower than during the calibra- 

 tion tests. 



The bismuth spiral was clamped securely in a holder that was capable 

 of being moved parallel to itself in various directions. In nearly all 

 cases the exciting current in the electro-magnet was 1.25 amperes and in 

 the few remaining cases the results have been corrected to this value. 

 Resistances were measured with a Wolff Wheatstone bridge and sensitive 

 galvanometer. 



The spiral received heat by radiation from the copper, and by con- 

 duction from the strong current of air when the disk was in motion. As 

 this made a direct determination of its temperature impossible, it was 

 decided to estimate the temperature from the resistance of the bismuth 

 when the magnetic field was off. This resistance was measured at fre- 

 quent intervals and the temperature computed with the aid of the resist- 

 ance temperature coefficient of bismuth. Thus, in a typical group of 

 observations at each position of the spiral, the following resistances were 

 observed: (1) magnetic field off, disk stationary; (2) magnetic field on, 

 disk stationary; (3) field on, disk running at two or more speeds in suc- 

 cession, in many cases repeating in reverse order; (4) field still on, disk 

 stationary; (5) field off , disk stationary. 



Allowance was made whenever necessary for the drift in temperature 

 between observations (1) and (5). In general, the mean of (1) and (5) 

 gave iv , the resistance of the spiral in a magnetic field of intensity 

 (practically) zero. The remaining observations gave values of w f , the 

 resistance with field on, at various speeds. In most cases the speeds were 

 0, 11, 60, and 112 revolutions per minute of the pedals, or 0, 36, 195, and 

 364 revolutions per minute of the disk. For each speed the value of 

 w f w was corrected for temperature, and from this the induction in 

 gausses was obtained from the calibration curve furnished with the spiral. 

 At the conclusion of each set of observations the spiral was advanced 

 a millimeter or so and the observations repeated. 



Most of the magnetic distortion was to be looked for along lines paral- 

 lel to the direction in which the portion of the disk between the poles was 

 moving, i.e., along the line AB in fig. 16. Nearly all of the observations 

 were accordingly made along this line and they will be considered first. 

 The results are shown in fig. 15, in which the abscissae represent dis- 

 tances in millimeters measured from the center of the field along the line 

 AB of fig. 16. Positive values lie in the direction in which the disk is 

 supposed to be rotating. The heavy vertical lines G G' in fig. 15 indi- 



